I have 3 reasons for playing mech sisters:
1. because I played sisters befoer codex WH and troops could not take transports, there was always occasions when you wanted them, so being able to get them is a bit like beeing given the sweets to the toy shop (not this dont make it the right thing to do or a better tactic, it is just a reaction)
2. And you will like this Plague: Flamers. The prefered weapon of Sisters, has an increddibly short range, if you are walking, it is too easey for your opponent to jump over it with his fast attack, and then pile his other troops in whilst the flamers are engaged. IF I use foot squads, which is rare, I take storm bolters as the special weapons, but they just dont give the same value for money as the flamers. Using rhinos allows for a super nasty mob handed drive by, as visible in any of my battle reps, when DG is applied to this, it shreds your opponent.
3. Painting, on average, it takes me up to 2 weeks to paint a vehicle, in the same time I can do less than 4 sisters. If I want 2000 points of sisters it would take be an eternity to paint an infantry army